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Hi,

A new problem seems to have emerged for me. Bought this car a year ago with approx 200k miles, did a bunch of service on it (TB cleaning but did not remove it from block, plugs, filter, trans oil, new o2 sensors) over the year. It ran like an absolute top. Loved driving it as my beater commuter car.

It started about 7 to 10 days after i changed o2 sensor (NTK) . When I first swapped the sensor it ran still perfect but maybe gas mileage was dropping a little bit (i track it religiously) .

at first it would just be one stumble at a red light on a 1 hour commute. Only notice because the car would just shake once. Then it got progressively worse over a week. No engine codes at all. No powerloss and when at RPM over 1500, it will drive absolutely 100% normal.

The symptom appears to only happen after the motor warms up On the code reader I can see rpm go from 620rpm (in drive idle) down to maybe 550 (with a jolt to the car), then just go back to normal. The hotter it gets the worse the symptoms become.

First thing after reading forums I did was clean IACV ( i however, did not replace oring seal). Relearned idle procedure ( right at 700rpm in neutral on my code reader) It seemed to make no difference.
I checked for vacuum leak (only by spray propane around areas of engine) and notice no drop in rpm.
I checked plugs and moved around the coil packs around. no difference.
I'm not loosing coolant or have coolant in oil. no smoke or sweet smell in exhaust, so I don't think (and hope) it's not the head gasket.
I burped the coolant (definitely had bubbles come out)
Battery is new. I cleaned all the ground contacts and verified that alternator output is good. Lights do not dim or anything when stuttering happens.
Put my original o2 sensors back on.
Listened to injectors with screwdriver and sounds rhythmic and consistent..
The MAP reads steady the whole time on my code reader as well as the intake temperature. I have no overheating problems and fans turn on when it should.



I really don't think it's a vacuum leak.
I have a feeling perhaps it's either a fuel delivery problem or headgasket
Maybe timing or valve clearance?

My car has a JDM D17 motor with no EGR valve.

Any advice would be wonderful
Thanks


Video here. If you turn on the audio in the video you can hear it stumbling/missing

https://imgur.com/a/PKrHk0l

 
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Changed PCV and Grommet.
Put old downstream o2 sensor back in.


No difference.

When I look at the STFT it doens't look like its trending down or up before missing.

It seems to only do it after reaching about 160deg coolant temp.

I have edited the title to remove the note about idle issue , since issue will also do it while on throttle under 2k rpm.

 
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Okay I figured out the problem, it was the Coil Pack on #4 Cylinder.

I did not go straight for coil packs since I made the assumption that if it was a true misfire , the ECU would trigger a misfire code. The ECU only triggered a code after I completely unplugged the a coil.

After noting that STFT seemed steady, I then started to think again either fuel pressure/injectors were off, or not sparking.

After I realized cyl 4 was giving me an issue , I ohm'ed the coil pack and showed 0 Ohms Resistance.

Wanted to update here in case anyone else had an issue. That originally started as stumbling only at idle in drive gear, to progressively getting worse when hotter.

Now i'm wondering if the ECU has an issue since it did not ever trigger me a misfire code, even though it was missing so bad. (up to 2 times a second)

 
Rep Power: 223

Hope you put denso coils in